This invention relates to a system for controlling a bank of parallel connected hot water heaters supplying hot water to a large institution or apartment complex. The hot water requirements in such installations varies widely during the day. For example, at midnight, one heater may easily handle the load while in the morning hours when people are preparing to go to work, it may take three or more continuously operating heaters. At other times, the load may be modulated between one or more heaters.
Excessive cycling of a hot water heater is known to increase the maintenance costs and to lead to premature failure. Excessive cycling promotes carbon buildup on the burners and combustion surfaces. This is true for both oil and gas fired heaters. Excessive cycling also stresses the ignition and safety circuits of the heater leading to premature failure and possible safety hazards.
Excessive cycling is also wasteful of energy. For increased energy efficiency, a heater should be operated as closely as possible at steady state conditions. In a plural heater system, this means that one heater should be selected as a lead heater and operated with the longest duty cycle while the remaining heaters should be selectively brought into operation as needed.
A number of systems have been invented to stage the heater operation in a system employing a bank of plural, parallel connected heaters. U.S. Pat. No. 3,964,466, issued Jun. 22, 1976, shows plural, parallel connected heaters having a thermostatic modulating valve at the output of each heater. If the temperature in a heater drops to a preset minimum, the thermostatic modulating valve closes, shifting the load to another heater in the bank. U.S. Pat. No. 3,461,854, issued Aug. 19, 1969, shows a bank of parallel connected heaters feeding a large hot water storage tank. The heaters are under control of a pair of thermostats which stage the operation by selectively operating the burners and pumps of the heaters.
While the above-mentioned patents do teach staging of a bank of parallel connected heaters, the prior art does not teach a staging system of simple design free of complicated thermostatic modulating valves and thermostatically controlled pumps.